1. Field of the Invention
The invention relates to a computer and display device which can be used in a hazardous (explosive and/or corrosive) environment for the input and output of operator and computer initiated data.
2. Description of the Prior Art
Computer control of processes producing or using explosive and/or corrosive gases, vapors, dusts, or liquids is currently accomplished by remotely locating the computer and/or display device in a control room situated in a non-hazardous area. This unnecessarily limits operator interaction with the process and creates system hazards since the operator can no longer see, hear, feel, or smell the process machinery or products. It has been industry's desire to have commercially available a computer and/or display device which would be certified safe for use in explosive/corrosive atmospheres, thereby placing the operator and the computer and display device directly at the process site. For the purposes of this disclosure, the computer and display device shall be referred to as a terminal.
To applicant's knowledge, no certified complete explosion-resistant process instruments or computer terminals are presently available. The user group, company, corporation, etc. normally assembles special instrumentation from a collection of previously qualified and certified components (for this disclosure consider the qualifying agent as Factory Mutual Research Corporation, Underwriters Laboratories or other nationally or internationally recognized certification agency). In that a computer terminal is a highly sophisticated device, no user group attempted to package or modify a device for hazardous environments.
Certain severe limitations immediately are evident. An explosion proof casting of sufficient volume to house a computer terminal would be too large and heavy to transport. The viewport or window in the explosion proof container would be very large and costly, and the thickness of the window would cause distortion when viewing the display. No effective means are currently available for operator data entry, i.e. a operator keyboard. The terminal must be protected from the environment, i.e. explosive/corrosive type substances and process-generated heat. The explosive process environment must be protected from the effects of the terminal, i.e. electrically generated sparks, excessively hot components, and static-generated discharges from the operator to the terminal, or from the CRT display (picture tube).
Plastic or rubber membrane keypanels have been refused certification because of their puncture susceptibility and the resulting possible erratic operation caused by exposed or corroded key contacts. Sealed key switches, per se, for use as data entry devices are already known in the prior art. For example, U.S. Pat. No. 4,339,634--Weber discloses a full travel rubber-gasketed switch which, however, could not be used in a hazardous environment because it could easily be punctured and also easily disabled or destroyed by corrosive agents. U.S. Pat. No. 3,383,487--Wiener discloses a two-part sealed metallic contact switch activated by deforming a plastic membrane by finger pressure or by electromagnets. Such technology could not be used in a hazardous environment since the contacts, though sealed, could cause sparking if the plastic membrane were punctured. Though mention was made of a sealed switch being acceptable in an explosive atmosphere, no certification of this type device has been found. This also pinpoints a common fallacy in much of the art in that an assumed or implied use in an hazardous explosive environment is made, whereas the only acceptable terminology is "Certified for Use in Explosive Environments" by some nationally or internationally recognized agency. U.S. Pat. No. 4,296,406--Peason describes a switch making use of variable resistance technology. This type of switch is inherently spark free; however, it still employs a flexible mylar (plastic) sheet which could be punctured, thereby creating an electrical safety hazard. Large areas of plastic, such as are required for alphanumeric keyboards, are subject to static electricity discharges. U.S. Pat. No. 4,066,853--Zenk describes a plastic membrane panel used in a matrix configuration comprised of conductive strips coated with a piezoresistant coating. Again, the panel is restricted to non-hazardous uses since it is easily punctured. International Publication Number W081/02223--Kroeger and Norquist also describes a flexible membrane pressure-sensitive type piezoelectric matrix keyboard. This design suffers from the common faults of (1) a plastic membrane which is easily punctured and (2) interaction between switches caused by the use of common subtrates for all switch or pressure points.